There have been broadly employed radiographic images such as X-ray images for diagnosis of the conditions of patients on the wards. Specifically, radiographic images using an intensifying-screen/film system have achieved enhancement of speed and image quality over its long history and are still used on the scene of medical treatment as an imaging system having high reliability and superior cost performance in combination. However, these image data are so-called analog image data, in which free image processing or instantaneous image transfer cannot be realized.
Recently, there appeared digital system radiographic image detection apparatuses, as typified by a computed radiography (also denoted simply as CR) and a flat panel detector (also denoted simply as FPD). In these apparatuses, digital radiographic images are obtained directly and can be displayed on an image display apparatus such as a cathode tube or liquid crystal panels, which renders it unnecessary to form images on photographic film. Accordingly, digital system radiographic image detection apparatuses have resulted in reduced necessities of image formation by a silver salt photographic system and leading to drastic improvement in convenience for diagnosis in hospitals or medical clinics.
The computed radiography (CR) as one of the digital technologies for radiographic imaging has been accepted mainly at medical sites. However, image sharpness is insufficient and spatial resolution is also insufficient, which have not yet reached the image quality level of the conventional screen/film system. Further, there appeared, as a digital X-ray imaging technology, an X-ray flat panel detector (FPD) using a thin film transistor (TFT), as described in, for example, the article “Amorphous Semiconductor Usher in Digital X-ray Imaging” described in Physics Today, November, 1997, page 24 and also in the article “Development of a High Resolution, Active Matrix, Flat-Panel Imager with Enhanced Fill Factor” described in SPIE, vol. 32, page 2 (1997).
To convert radiation to visible light is employed a scintillator panel made of an X-ray phosphor which is emissive for radiation. The use of a scintillator panel exhibiting enhanced emission efficiency is necessary for enhancement of the SN ratio in radiography at a relatively low dose. Generally, the emission efficiency of a scintillator panel depends of the phosphor layer thickness and X-ray absorbance of the phosphor. A thicker phosphor layer causes more scattering of emission within the phosphor layer, leading to deteriorated sharpness. Accordingly, necessary sharpness for desired image quality level necessarily determines the layer thickness.
Specifically, cesium iodide (CsI) exhibits a relatively high conversion rate of from X-rays to visible light. Further, a columnar crystal structure of the phosphor can readily be formed through vapor deposition and its light guide effect inhibits scattering of emitted light within the crystal, enabling an increase of the phosphor layer thickness. However, the use of CsI alone results in reduced emission efficiency. For example, JP-B 54-35060 (hereinafter, the term JP-B refers to Japanese Patent Publication) disclosed a technique for use as an X-ray phosphor in which a mixture of CsI and sodium iodide (NaI) at any mixing ratio was deposited on a substrate to form sodium-activated cesium iodide (CsI:Na), which was further subjected to annealing as a post-treatment to achieve enhanced visible-conversion efficiency.
However, a scintillator (phosphor layer) based on CsI exhibits deliquescence and suffered a disadvantage of characteristics being deteriorated with aging. Accordingly, to prevent such deterioration with aging, there is proposed formation of a moisture-proof protective layer provided on the surface of the scintillator (phosphor layer) based on CsI. For instance, there is known a technique in which the upper portion and the side portion of a scintillator layer (corresponding to a phosphor layer of the present invention) and the circumferential portion of the scintillator layer on a substrate are covered with a poly-p-xylilene resin (as described in, for example, patent document 1). There is also known a technique of covering at least the opposite side of a scintillator layer to the side opposing to the support and a side surface with a transparent resin film exhibiting a moisture permeability of less than 1.2 g/m2·day (as described in, for example, patent document 2). Enhanced moisture-proofing is achieved by these protective layers.
Generally, in cases when placing a scintillator panel on a flat light receiving element, a cushioning material is provided between the protective cover and the scintillator panel, and the scintillator panel is pressed at an optimal pressure onto the light receiving element through the pressure of the compressed cushioning material when the protective cover is provided. Accordingly, when assembling a flat panel, a scintillator panel and a cushioning material are sequentially placed on a light receiving element disposed in a housing and thereafter, a protective cover is fixed to the housing by screws or the like.
In this case, excessively strong pressure of the cushioning material breaks the top portion of phosphor crystals having a columnar crystal structure, resulting in lowered contrast of a radiation image. To the contrary, in cases of a weak pressure of the cushioning material, a displacement between the scintillator panel surface and the flat light receiving element surface is caused when turning the FPD downward or by vibration, resulting in reduced signal correction precision at the individual elements of a flat light receiving element and leading to deteriorated graininess or sharpness of the obtained image. There is also produced problems such that friction between a scintillator panel and a flat light receiving element, caused by movement or vibration of the FPD, tends to produce defects in the flat light receiving element or the phosphor layer.
In general, the thickness of a phosphor layer needs to be not less than 400 μm to obtain a radiation image of enhanced graininess; however, the increased mass of the scintillator panel, due to such an increased layer thickness or an increased panel size has pronounced the foregoing problems.
To overcome such problems, there has been proposed a method of securing a scintillator panel and a flat light receiving element with an adhesive (as described in, for example, patent document 1) or a method of pasting them together with a matching oil (as described in, for example, patent document 2), but producing problems such as occurrence of unevenness due to the adhesive or the matching oil, or the increased number of work steps. Further, overhaul of a FDP or replacement of a scintillator panel is impossible in these methods, producing serious problems in maintenance.
Production of scintillator plates through a gas phase method were generally conducted by forming a scintillator layer on a rigid substrate such as aluminum or amorphous carbon and covering the entire surface of the scintillator layer with a protective layer (as described in Japanese Patent No. 3566926). However, formation of a scintillator layer on a substrate which cannot be freely bent is easily affected by deformation of the substrate or curvature at the time of vapor deposition when sticking a scintillator plate on the flat light-receiving element surface with paste, leading to defects such that uniform image quality characteristics cannot be achieved with the flat light-receiving surface of a flat panel detector. Accordingly, such problems have become serious along with the recent trend of increasingly larger flat panel detectors.
To avoid these problems was generally performed formation of a scintillator directly on an imaging device through vapor deposition or the use of a medical intensifying screen exhibiting flexibility but low sharpness instead of a scintillator.
In view of the foregoing situation, there has been desired development of a radiation flat panel detector which is suitable for production, prevents aging deterioration of characteristics of the phosphor layer, protects the phosphor layer from chemical alteration or physical impact and maintains the stable contact state between the scintillator panel and the flat light receiving element.
Patent document 1: JP 2006-189377 A
Patent document 2: JP 2000-9845 A